Make function have ellipsis for arguments in help() function

If you type help(vars), the following is produced:

vars(...)
    vars([object]) -> dictionary

    Without arguments, equivalent to locals().
    With an argument, equivalent to object.__dict__.

When I do the following:

def func(x, y): pass

help(func)

it displays this:

func(x, y)

How can I change it so that it shows up with ... between the parentheses like the built-in function vars()? (That is, func(...))

Edit: It has been suggested to use a docstring, but that won't do what I want. Here is an example:

def func(x, y):
    """func(...) -> None"""

help(func)

result:

func(x, y)
    func(...) -> None

You see, x, y is still being displayed instead of ...

Solution

You have (at least) two alternatives to achieve what you want.

The best alternative would be to override the __str__ method of the inspect.Signature class. However, as it is written in C, it is read only.

So to do that you need to extend the class as following:

class MySignature(inspect.Signature):
  def __str__(self):
    return '(...)'

then after defining your function you execute:

func_signature = inspect.signature(func)
func.__signature__ = MySignature(func_signature.parameters.values(), 
                                 return_annotation=func_signature.return_annotation)

which would then return the following for help(func):

Help on function func in module __main__:

func(...)
(END)

With this approach inspect.signature still works:

In [1]: inspect.signature(func)
Out[1]: <MySignature (...)>

Alternatively if you don't really care about being able to properly introspect your function (and probably some other use cases), then you can define the value of your function's __signature__ to an object which is not a Signature instance:

def func(x, y):
    pass

func.__signature__ = object()

help(func)

generates the result:

Help on function func in module __main__:

func(...)
(END)

But now inspect.signature(func) will raise TypeError: unexpected object <object object at 0x10182e200> in __signature__ attribute.

Note: this last version is quite hacky and I would not recommend it.

For more info on these two techniques and how the signature works see PEP 0362.

Update: For python 2.7 you can do the following (probably better using a mock framework):

In [1]: import inspect

In [2]: def myformatargspec(*args, **kwargs):
   ...:     return '(...)'
   ...:

In [3]: def func(x, y):
   ...:     pass
   ...: 

In [6]: inspect.formatargspec = myformatargspec

In [7]: help(func)

Help on function func in module __main__:

func(...)
(END)